Petunias provide large showy blooms on compact plants. These summer annuals are prized in hanging baskets where their big blooms provide a mound of bright color nestled amongst green foliage. Growing petunias from seed allows you to design your own hanging basket displays. Plant a single color of petunias for a bold color statement or combine two or more complementary colors and add a rainbow of blooms to your garden design. Combine the standard upright varieties with some cascading petunias around the edge of the basket to create a vertical display of flowers.
Fill a seed-starting flat with a fine-textured potting medium. Water the potting medium until it is evenly moist but not soggy.
Sprinkle the petunia seeds on top the potting soil. Apply the small, fine seeds as thinly as possible, planting only two to three seeds per square inch.
Cover the flat with a piece of clear plastic wrap. Set the flat in a 70 to 85 degree F area to germinate. Place the flat so it receives bright, indirect light as petunias require light to germinate.
Remove the plastic wrap once the seeds sprout. Move the flat to a brightly lit, 60 to 65 F room. Water when the soil surface begins to feel dry.
Thin the seedlings once their leaves are fully open. Pluck out the extra seedlings so the petunias are spaced approximately 2 inches apart in all directions. Alternately, transplant the petunias into individual 3-inch diameter pots, planting them at the same depth in the pot that they were at in the flat.
Fertilize the petunia seedlings every two weeks. Apply a one-quarter to one-half strength dilution of soluble houseplant fertilizer.
Planting the Basket
Fill the hanging basket with a well-draining potting mix to within 2 inches of the rim. Water the mix until water begins draining from the bottom of the basket.
Plant the petunia seedlings into the basket at the same depth they were growing at in the seedling pots. Space the petunias 3 to 4 inches apart in all directions, planting larger varieties in the center of the basket and smaller types or trailing types around the edge.
Hang the basket in an area that receives at least six hours of sunlight a day. Water the basket when the soil surface begins to feel dry.
Fertilize the petunias once a month. Apply a soluble 1:2:1 ratio fertilizer at the rate recommended on the product label.
Pinch off the petunia blossoms when they begin to fade. This process, called deadheading, encourages further blooms and keeps the basket looking its best.